A high-profile child killer decided to kill another inmate at New Zealand's toughest prison because he didn't like the man - and spent a month planning his attack.

LISTEN ABOVE: New Zealand Herald senior crime reporter Anna Leask spoke to Larry Williams about the case

But much to the killer's disappointment, his victim, in spite of being stabbed with a fluorescent lightbulb and then a broom handle, lived.

Stephen Roger Williams was today sentenced to preventive detention on top of his existing life sentence for the attempted murder of Nikki Roper at Auckland Prison, also known as Paremoremo, on December 16.

He appeared before Justice Kit Toogood at 9am.

Williams and Roper are serving life sentences for murder.

Williams murdered Coral, his stepdaughter, in 2003.

He beat the 6-year-old to death and dumped her body near Featherston.

Roper was jailed for life with a minimum non-parole period of 14 years for murdering Christchurch mother Alexsis Tovizi in December 2010.

Williams was sentenced in the High Court at Auckland today after pleading guilty to attempted murder in January.

The time will be served concurrently with his murder sentence.

Crown prosecutor Scott McColgan revealed in court today that Williams had 96 convictions - the count of attempted murder brought his tally to 97.

Williams, appearing in the dock in a grey T-shirt and covered in tattoos including on the back of his shaved head, made it clear that he didn't wish to take legal advice and wanted to represent himself in court.

Before sentencing the Herald was granted access to the police summary of facts, which outlined Williams' attack on Roper.

Both killers were housed in the prison's C Block and at about 2pm on December 11 Roper entered Williams cell to get his back tattooed.

Tattooing equipment is illegal in prisons but Williams had tattooed Roper previously and he had made an "appointment" for another session.

Roper sat down on a chair in Williams' cell with his back to the older killer.

"(Williams) has put down the tattoo gun and picked up a long fluorescent light tube, which he had removed from the light in his room earlier in the day," the summary stated.

"He has stabbed the complainant in the neck area causing the light tube into the wound of (Roper's) neck.

Williams then tried to choke Roper, putting pressure on the injured man's carotid artery in his neck.

The pair struggled and Roper made his way onto the landing outside Williams' cell where security cameras captured the ongoing assault.

Roper lost consciousness and Williams continued the vicious attack, stomping on his head.

"He has then picked up a broom and hit (Roper) across the head area with it," the summary revealed.

"The broom has broken, however (Williams) has continued to assault (Roper) with the broom handle.

"(Williams) has then tried to stab (Roper) with the sharp end of the broom handle in the back of his neck area."

Williams carried on beating Roper until prison guards got to the landing and stopped the attack.

At that stage Roper was unresponsive and rushed to Auckland City Hospital in a serious condition.

Roper suffered serious injuries and had to undergo emergency surgery to save his life.

Police listed Roper's injuries in the summary, including a 30cm gash to his head and neck area which required 34 staples to close it; a fractured eye socket, cuts and bruising to his hands, a cut to his left shoulder and a second laceration to the back of the head.

Roper recovered from his injuries and was transported back to the prison, where he remains.

After his arrest Williams told police that he stabbed Roper because he "doesn't like" him.

"He had been planning the attack for a month," the summary said.

"He wanted to kill him and can't believe he's still alive."

The Crown sought a sentence of preventive detention for Williams, saying it was the only appropriate outcome to ensure the safety of the community.

McColgan said there were eight aggravating factors to the offending.

They were extreme violence; premeditation; serious injury; the use of weapons; attacking Roper's head; an element of "vigilante retribution" as Williams had decided he did not like his victim and considered him a "snitch"; vulnerability in that Roper was on the landing "effectively locked in and at the mercy of Williams until Corrections had an appropriate number of staff" to intervene.

"This was a sustained and ferocious attack on the victim that was only stopped by the intervention of Corrections' staff," said McColgan.

He said there was a "scrambling and panicked attempt to escape" by Roper and if not for Corrections staff stopping the attack, Williams would have been successful in his murderous plan.

"He presents a very serious risk of further violent offending," McColgan said.

"Preventive detention is necessary... He has shifted into the type of offender that needs to be looked at in terms of his risk to others."

The court heard that reports from health assessors supported the conclusion that Williams posed a threat of "imminent danger to others".

McColgan said his criminal history showed a pattern of "serious violent offending".

"The seriousness of the harm caused to the community Williams and his offending is significant and is escalating, in effect, while he is in prison," he said.

"The heath assessors reports... credible information indication a tendency by Williams to commit serious violent offences in the future.

"Indeed he has been very clear about that fact that he intends to continue harming others until he receives the sentence he should have got for his original murder conviction.

McColgan said in an interview with police Williams "makes it quite clear that prison is not a deterrent for him and that he appreciates himself that should he ever be released he is likely to go on and hurt further people".

When asked by Justice Toogood if he wanted address the court on sentencing, Williams responded "nah".

"I just want to get sentenced and get out of here," he said.

Justice Toogood, in sentencing Williams, said the killer "was annoyed" when he learned he had not killed Roper in the prison attack.

"Indeed you expressed regret and disappointment that he did not die," he said.

"The brutality of the attack and your indifference to Mr Roper... is chilling and disturbing."

Justice Toogood said there was a degree of "vigilantism and retribution" given Williams believed Roper was "a nark" and had "snitched" on him.

He also revealed that in 2005 Williams was brought to court on charges of wounding another inmate with a sharpened toothbrush.

Justice Toogood outlined Williams life - a life punctuated with drug abuse, violence, gang association and broken relationships.

He said the murderer was "completely institutionalised" and had indicated he never wanted to be released from prison nor would he ever seek parole.

Justice Toogood said he had to consider Williams' clear pattern of serious offending including 16 convictions for violent offending two of which were racked up after he was jailed for Coral's murder.

He said Williams attacks undermined the safety of New Zealand prisons and the man "made it clear" that he intended to act violently.

"And even if you can, to kill in the future," Justice Toogood said.

"It seems to me that you are a very disturbed individual."

He sentenced Williams to 10 years in prison for the attempted murder with preventive detention.

Roper is no stranger to attacks by other inmates.

IN July 2015 he was hospitalised after a severe beating in an exercise yard at Auckland Prison.

He was taken to hospital after that attack in a critical condition

- NZ Herald